This invention relates generally to a hydraulic tensioner having a piston longitudinally movable in a housing and more particularly to a check valve seat for such a tensioner with a vent path on the bottom surface of the check valve seat. This invention also relates to a tensioner whose components are made from anodized aluminum.
Tensioning devices, such as hydraulic tensioners, are used as a control device for power transmission chains as the chain travels between a plurality of sprockets. In an automotive application, the tension of the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. As a result, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or un-meshing of the chain teeth. It is especially important in the case of a chain-driven camshaft in an internal combustion engine to prevent the chain from slipping because the camshaft timing can be misaligned by several degrees, possibly rendering the engine inoperative or causing damage.
A potential problem with hydraulic tensioners, however, is that the fluid pressure inside of the fluid chamber formed between the hollow piston and the bore in the housing may change due to the introduction of air into the fluid chamber. If an excessive amount of air is present on the chamber, the piston will easily move due to the compressibility of the air. As a result, the proper functioning of the tensioner will not be achieved. It is, therefore, desirable to provide for venting of air from the hydraulic fluid chamber.
Venting involves allowing air to escape from the high pressure chamber in the piston. One method for venting air is disclosed in U.S. Pat. No. 4,507,103, where a plug having a groove machined onto its flank is pressed into a smooth bore at the top of the piston so that a connection exists between the atmosphere and the fluid reservoir. A drawback to this method is that the plug and groove must be properly machined to fit into the smooth bore and to provide a suitable connection.
Another method for venting air is disclosed in U.S. Pat. No. 5,346,436, where the air vent is a disc having at least one channel on at least one surface to provide a connection between the atmosphere and the fluid reservoir. The method of venting air disclosed in 5,346,436, however, refers to a primary tensioner where the piston protrudes upward, and the air vent, therefore, is at the top of the piston.
The present invention is directed toward a secondary tensioner where the tensioner may be placed in a variety of orientations, including one in which the piston extends down when extending out from the bore. In such a secondary tensioner, an air vent at the top of the piston will not assist in the venting of air. The check valve vent of the present invention is a simple and inexpensive apparatus to provide both an air vent for a secondary hydraulic tensioner and a pliable seat for a check valve. The check valve assembly sits within the seat of the check valve vent, which has a tortuous air vent path at the bottom. In this way, the seat allows the check valve to seal effectively against back pressure, while at the same time air trapped in the piston can vent along the tortuous path.
Another problem common to hydraulic tensioners is the fit between the piston and the bore. While the gap between piston and bore must be kept small to minimize fluid leakage, the piston can often stick to the bore and not slide properly. A hydraulic tensioner having an aluminum piston sliding in an aluminum housing often will not perform properly because of sticking between the components. Moreover, the aluminum components are extremely susceptible to wear and microscopic friction welding. It is, therefore, desirable to prevent the aluminum components from sticking and to protect them from wear.